The present invention relates to a plastic bowling pin and a method for manufacturing the plastic bowling pin.
In general, the bowling pin has been manufactured by cutting a blank of laminated wood into a original bowling pin configuration and then by applying synthetic resin on the surface of the cut-out pin. In use, the bowling pin is damaged by impacts during the game and thus it is necessary to repair the damaged pins or discard them by burning.
In the wooden bowling pin, it has been discarded in burning only the wood portion thereof after the synthetic resin was cut and removed from the pin surface since the burning of the synthetic resin applied on the pin surface would generate harmful substances causing air pollution, or it has been regenerated with applying again new synthetic resin on the damaged surface. However, the cutting of only the synthetic resin from the pin surface is cumbersome and increases the discarding cost or the regeneration cost.
In view of these actual circumstances, it has been proposed to form whole a bowling pin of synthetic resin. According to such a plastic bowling pin, it is possible to regenerate the damaged pin, eliminate the discarding cost and reduce the regenerating cost since the damaged bowling pin of synthetic resin can be wholly melted and regenerated again to a new bowling pin by forcing the molten resin to be flowed into a mold for forming the bowling pin.
However, there are following problems in conventional plastic bowling pins.
Since the bowling pin suffers from severe impact in its use, it must be made of plastic material having high durability. The synthetic resin of high durability is generally expensive and therefore it would be enormously expensive if whole the bowling pin is made of such plastic material of high durability. Accordingly, the plastic bowling pin is usually formed of two portions, that is, a core portion made of cheap plastic material of general purpose and a skin layer covering the core portion made of special plastic material having high durability.
In order to form the skin layer on the core portion by using synthetic resin different from that of the core portion, it is necessary to use both a mold for molding the core portion and a mold for molding the skin layer. In this case, it is necessary to form the skin layer so that it has a substantially constant thickness around the molded core portion and therefore there is a problem that the exact positioning of the molded core portion relative to the mold for molding the skin layer is very difficult.
On the other hand, when the whole bowling pin is formed of synthetic resin there is a problem that it is difficult to obtain exact configuration, weight etc. since the massive synthetic resin forming the bowling pin often tends to leave sink marks on its surface. Since the bowling pin is ruled in detail in its specification as to not only its configuration, but its weight, the position of the center of gravity and the bounce property, a bowling pin having deficiencies in the configuration etc. caused by the sink marks cannot be used in playing games.
It is therefore an object of the present invention to provide a plastic bowling pin and method for manufacturing the bowling pin in which exact configuration etc. defined in the rules can be obtained with avoiding deficiencies such as sink marks during the molding process as well as improving the efficiency or operability in molding the bowling pin.
According to the present invention, a method for manufacturing a plastic bowling pin includes using a first mold having a first molding cavity of a smaller dimension than the predetermined outside dimension of the bowling pin and having first axial rods each having a predetermined diameter and displaceable into and from the first molding cavity along a longitudinal axis. A second mold is used that has a second molding cavity of a substantially same dimension as the outside dimension of the bowling pin and having second axial rods each having a predetermined diameter and displaceable into and from the second molding cavity along a longitudinal axis. The method comprises steps of; molding a core portion of the bowling pin by forcing molten synthetic resin to be flowed into the first molding cavity of the first mold after the first axial rods have been inserted into the first molding cavity; removing the first axial rods therefrom after the synthetic resin has been solidified; molding a skin layer on the external surface of the molded core portion by forcing molten synthetic resin to be flowed into a space between the external surface of the molded core portion and the second molding cavity of the second mold after the molded core portion has been positioned relative to the second molding cavity with inserting the second axial rods into an aperture formed in the core portion by the first axial rods.
In this method, since the apertures are formed by the first axial rods in the core portion both on the top and bottom ends, it is possible to reduce the mass of the core portion and it is also possible to adjust the weight etc. by controlling the dimension of the apertures. Accordingly, it is possible to obtain exact configuration etc. defined in the rules with avoiding deficiencies such as sink marks during the molding process.
Also according to the invention, since the second axial rods are inserted into the apertures formed by the first axial rods during the molding of the skin layer, it is possible to achieve the exact positioning of the core portion relative to the second mold during the molding of the skin layer and thus to improve the operability or efficiency in molding the bowling pin.
According to the present invention the method further comprising a step of press fitting plastic plugs into the apertures formed in the bowling pin by the first and second axial rods after the skin layer has been molded.
In this method, since the plastic plug can be easily removed from the bowling pin, it is possible to reuse the removed plug to the regenerated bowling pin.
According to the present invention, the first and second axial rods are respectively formed by a pair of bar members each displaceable into and from the opposite ends of the first and second molding cavities of the first and second molds.
In this method, since the apertures are formed by the first axial rods in the core portion both on the top and bottom ends thereof, it is possible to reduce the mass of the core portion at regions other than the constricted portion and thus effectively prevent sink marks. Also since the second axial rods are inserted to the apertures formed on the top and bottom ends, it is possible to further improve the accuracy of positioning of the core portion relative to the second molding cavity of the second mold.
According to the present invention, the pair of bar members of the first axial rods are adapted to be abutted at their tip ends, and the core portion molding step is carried out under a condition of the tip ends being abutted.
In this method, since a through aperture is formed in the core portion from the top end to the bottom end thereof, it is possible to reduce the mass of the core portion in all regions including the constricted portion.
According to the present invention, the forced flow of the molten synthetic resin into the first and second molding cavities is carried out by injection, and the injection in the skin layer molding step is carried out at a position out of the center of gravity of the bowling pin.
In usual, it is recognized that the synthetic resin near the injection port is cooled and solidified later than that of other portions and thus has lesser rigidity than that of the synthetic resin forming other portions. On the other hand, the bowling pin is struck at the position of the center of gravity and suffers from an excessive impact at that position. Accordingly, if the position of the injection port would be arranged at a position corresponding to the center of gravity, the rigidity of the bowling pin thereat will be reduced and thus the life time of the bowling pin will be also reduced. Accordingly, since the injection in the skin layer molding step is carried out at a position out of the center of gravity of the bowling pin, it is possible to ensure the rigidity of the bowling pin at the position of the center of gravity and maintain a long life time of the bowling pin.
According to the present invention, the bowling pin has apertures within a core portion of the bowling pin formed by the first axial rods as well as an opening of the skin layer of the bowling pin formed by the second axial rods.
In this bowling pin, since it has the apertures within the core portion formed by the first axial rods and the opening of the aperture in the skin layer formed by the second axial rods, it is possible to provide a high quality plastic bowling pin in which its whole weight exactly corresponds to the ruled weight and there is not any sink mark on its surface.